1. Field of the Invention
The present invention relates to a circuit board material with a resistance layer useful for the production of a printed resistance circuit board, more particularly relates to a plating bath for forming a resistance layer on a conductive base forming that circuit board material with a resistance layer by plating and a plating method of the same.
2. Description of the Related Art
A printed circuit board material including resistors (hereinafter called a “resistance circuit board material”) is generally provided in the form of a multilayer body of an insulating substrate and a conductive base with a resistance layer comprised of a resistance layer bonded with that substrate and copper foil or another high conductivity base bonded with that resistance layer.
A printed resistance circuit using a resistance circuit board material is produced by the subtractive method giving insulating regions from which all of the resistance layer and conductive base on the insulating substrate are removed, resistance regions from which the high conductivity base is removed, and all remaining conductive regions in accordance with the targeted pattern of the circuit (mask etching).
In the past, as the material forming the resistance layer, a carbon-based resistance material was generally used. As other metal thin films used, nickel electroplating including phosphorus (Japanese Patent Publication (A) No. 48-73762 and Japanese National Publication No. 63-500133), nickel electroplating including tin (Japanese Patent Publication (A) No. 54-72468), etc. have been proposed. With these types of metal thin resistance layers, however, while it is possible to obtain a film with a high sheet resistance by reducing the thickness, in general, if the thickness is reduced, the uniformity of the metal film is lost and a constant sheet resistance cannot be obtained, so there are limits to the reduction of thickness.
That is, in the production of a conductive base with a resistance layer, a thin resistance layer is formed on a conductive base by electroplating, but to raise the bonding strength of the conductive base with a resistance layer with the insulating substrate, the surface of the conductive base is roughened, then is plated with the Ni—P etc. serving as resistance layer. With this method, however, since the resistance layer is present on the rough surface of the conductive base, in particular roughened by fine roughened particles, even right after plating, the uniformity of thickness of the plating is poor and the sheet resistance lacks stability.
Further, dissolution of part of the resistance layer cannot be avoided since the layer of the conductive base is etched away when used as a resistance circuit board material. Further, if there is unevenness in thickness in the Ni—P plating resistance layer, there is the defect that even part of the resistance layer is sacrificed in order to completely remove the layer of the conductive base. It was extremely difficult to stably leave resistors and produce a printed resistance circuit board. Further, when producing a multilayer printed resistance circuit board, the printed resistance circuit board is hot pressed. There are therefore the defects that at this time, cracks occur in parts of only the resistance layer (parts where conductive base is etched away), the resistance increases, or sometimes the circuit becomes open.
In forming a resistance layer by such Ni—P alloy plating of the above patent publications, nickel ions, phosphorous acid ions, and phosphoric acid ions are essential. A plating bath for forming the resistance layer of the former patent publication also includes sulfuric acid ions and chlorine ions. A conductive base with a resistance layer obtained by plating a conductive base by such a bath suffers from uneven color at the time of plating and variations in the plating layer microscopically. In a wide material used at the time of mass production (for example, having a width greater than 300 mm), variation easily occurs in the plating thickness and phosphorus content in the width direction and the fluctuation in resistance of the resistance circuit becomes greater.
In the case of a resistance layer made of an Ni—Sn alloy, tin oxides or hydroxides remain on the insulating substrate when etching the resistance layer when forming insulation regions (dissolving the Ni—Sn) and the problem arises of poor insulation. Further, Ni—Cr, Ni—Cr—Al—Si, etc. formed by vapor deposition have been developed for the same purposes, but problems of cost and productivity and also problems of a low bonding strength with the insulating material have been pointed out.